LMSimilarity.DefaultCollectionModel, an implementation of the former, that
computes the term probability as the number of occurrences of the term in the
collection, divided by the total number of tokens.

WARNING: This API is experimental and might change in incompatible ways in the next release.

explain

Subclasses should implement this method to explain the score. expl
already contains the score, the name of the class and the doc id, as well
as the term frequency and its explanation; subclasses can add additional
clauses to explain details of their scoring formulae.